Basically (and yes, I am looking at it from an American perspective), I
argue that like all Americans, interaction designers are embarassed to talk
about class, even though it's implicit in so much of what we do. I've never
seen a user persona that explicitly designates a user as "low class" or
"upper class", using those terms in the hard manner we usually mean them in
common language. I've had meetings where my client has told me that their
customers are "rich snobs", and I've also had clients who openly referred to
one segment of their customers as "white trash". In both cases, this
knowledge was both business-strategy-appropriate and critical to the site's
design approach.

Class comes into play most obviously in a site's visual design. For example
the "cultural elite" or the "creative classes" may think that Comic Sans
with drop shadows is the ultimate in bad design, even though millions of
other (lower class) Americans think that's the cat's meow. And there are
many who think that Helvetica is the ugliest, most boring font ever invented
and utterly inappropriate for any web site, even though the design elite
generally loves it. The photography on a site speaks a lot to class,
obviously, but so does color, design density, illustration, texture,
"voice", and a million other aspects of design.

So what *interaction design* strategies are influenced by class? Do
lower-class users who are used to getting ripped off have a lower
expectation of customer service? Is it more acceptable to ask for an email
opt-in from one class versus another? Does an indication of price on a page
turn off upper class users? Does your site speak to your user's class
ambitions? I think of Peter Morville's honeycomb diagram
(http://tinyurl.com/o8tnr) of UX design and I can easily see class being a
major factor in every one of those hexagons.